Saturated linear polyesters such as polyethylene terephthalate (hereinafter referred to as PETP) are, owing to their excellent physico-chemical properties such as mechanical strength, heat and chemical resistance, transparancy etc., nowadays used for various applications such as packaging materials, electric insulating materials, fibres and filaments as well as support material for magnetic tapes and photographic films.
In view of their widespread use, the processes for manufacturing PETP-granulate and converting said PETP-granulate to film are well known to those skilled in the art.
PETP-film materials are generally obtained by melt-extrusion of the polyester through an extruder, rapidly cooling the-melt-extruded sheet on the surface of a cooling drum and then biaxially orienting the solidified material in longitudinal and transverse direction.
The polyester which is melt extruded can be prepared according to two main methods.
In a first method there is an ester exchange reaction between dimethyl terephthalate (DMT) and ethylene glycol (EG) to form bis(/-hydroxyethyl-terephthalate (BHET) whereupon polycondensation takes place. Both in the ester exchange reaction and in the polycondensation reaction catalysts are used : e.g. a zinc, magnesium, manganese, or cobalt salt or mixtures thereof in particular manganese acetate as ester exchange catalyst, and antimony compounds and/or germanium compounds (such as antimony and/or germanium dioxide) as polycondensation catalyst, and stabilisers such as e.g. phosphorus compounds.
In a second method the BHET is obtained by direct esterification of terephthalic acid (PTA) with ethylene glycol (EG). Antimony trioxide and/or germanium dioxide are also added as polycondensation catalyst and a phosphorus compound may be added as stabilizer.
The above processes may further be executed in either a batch or a continuous reactor system. Particulars about the direct esterification method are described in e.g. EU-A-0105 522 and EU-A-0159 817. Particulars about the first PETP production method (the ester exchange reaction method) may be found e.g. in GB A 1 221 788, GB-A-1 274 858, GB-A-1 108 096, GB-A-1 185 984 and GB-A-1,091 234.
Particulars about the film-forming process of PETP starting from PETP-granulate may be found in GB-A-1 269 127, GB-1-1 312 263 and EU-A-0 022 278.
During the film forming process the PETP-polymer is during a substantial period of time kept at elevated temperature and in contact with the surrounding air. According to a conventional process the PETP-polymer produced according to e.g. a continuous direct esterification and polycondensation process is after leaving the finishing polymerisation reactor quenched in a cooling bath, cutted to granules and pneumatically transported to the supply silos of the PETP-film extrusion plant. From these supply silos PETP-granulate is then pneumatically transported to the abovementioned dryers, and then further heated in an extrusion apparatus to PETP-melt suitable after filtration, for being melt-extruded. According to a more convenient and preferential way of working the PETP-melt such as produced in e.g. the continuous direct esterification and polycondensation process is directly fed in melt-form to the PETP-film extrusion apparatus.
It is apparent from the above description that the thermo-oxydative stability of the PETP-produced is an essential and critical requirement for producing PETP-film meeting high standards of quality. The latter applies in particular to PETP-polymer which should be melt-extruded for the production of PETP-films suitable as support for photographic films. One of the essential requirements set forth for PETP-films used as support for photographic materials is the clarity and the transparancy of the produced PETP-film. The transparancy of the PETP-film produced is determined to a large extent by controlling the crystallinity of the film during melt-extrusion. In particular during melt-extrusion a good heat transfer between the extruded PETP-film and the moving quenching drum should be realised. The method of electrostatically adhering the melt-extruded film to the quenching drum as disclosed in U.S. Pat. No. 4,310,294 may be applied so as to become a substantially amorphous film suitable for further processing, Including longitudinal stretching.
Another essential requirement however for obtaining clear transparent polyester film supports is the thermo-oxydative stability of the PETP-polymer itself. Knowing that the clarity and transparency of a polyester film sheet is enhanced by increasing the thermo-oxydative stability of the PETP-polymer various efforts have been performed to increase said PETP thermal stability by modifying the polyester used for the preparation of the PETP-film. Said efforts particularly are aimed to incorporate into the PETP-polymer stabilizing compounds during either extrusion or during the PETP-production process.
The German Patent Publication 1 152 259 discloses e.g. the incorporation of phosphoric acids or alkyl or aryl ester derivatives hereof for stabilising PETP. As examples of such compounds are mentioned triarylphosphates such as triphenylphosphate, trialkylphosphate such as triethylphosphate, tributylphosphate, and also partially esterified phosphoric acid derivatives such as dibutyl or diphenylphosphate.
The Belgian Patent no. 851,082 discloses the use of a mixture of triarylphosphites and phenolic compounds for heat stabilising various polymers including PETP.
The Belgian Patent no. 641,074 discloses the use of phosphonate compounds suitable for the oxidative stability of polyesters.
U.S. Pat. No. 4 385,145 discloses the stabilisation of poly(alkylene terephthalate) compositions by incorporation of bis-(alkylphenyl) pentaerythritol diphosphite ester. A particularly suitable stabiliser is bis-(2,4-di-t-butylphenyl) pentaerythritol diphosphonite, commercially available under the trade name Ultranox 626 from Borg Warner Chemicals Inc., Parkersburg W. Va, USA.
Research Disclosure no. 14434, published Apr. 1976 discloses a synergistic mixture of crosslinking inhibitor compounds comprising at least one phosphoric acid ester and at least one hindered phenolic compound to reduce color formation and to improve other physical properties of certain polyesters.
A particularly preferred example of hindered phenolic compounds cited is e.g. tetrakis-(methylene 3-(4'-hydroxy -3',5'-di-t-butylphenyl proprionate) methane commercially available under the trademark Irganox 1010 from Ciba-Geigy AG. Switzerland. Examples of preferred phosphorus acid esters are the fully esterified ester forms such as trialkyl, phenyl and alkylphenyl ester forms. EU-A-0 238 140 discloses a polyester resin composition particularly excellent in heat resistance comprising a crystalline polyester resin wherein a mixture of a hindered phenolic antioxidant and at least one antioxidant selected from phosphorus containing antioxidants and sulfur containing antioxidants is incorporated.
An example of such a mixture is e.g. the abovementioned tetrakis (methylene 3-(4'-hydroxy-3',5'-di-t-butylphenyl) proprionate) methane, and distearyl pentaerythritol diphosphite, commercially available under the trade name Weston 618 from Borg-Warner Corp., USA
WO 88/01285 discloses the stabilization of thermoforming polyester compositions by using a combination of hindered phenolic antioxidant such as the compound marketed by Ciba-Geigy under the trade mark Irganox 1010, abovementioned, and phosphorus compounds such as phosphites and phosphonites including e.g. diphenyl phosphite, tristearylphosphite, and the compounds abovementioned presently marketed under the trade marks Ultranox 626 and Weston 618.
JP Kokai 76,111,858 disclose the stabilization of heat- and fire resistant polyester moldings by using a mixture of tri-methyl-phosphate and di-ethyl (3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate, commercially available under the trade name Irganox 1222 from Ciba-Geigy AG.
In the article 'Effect of certain stabilizers on the thermal oxidation of poly(ethylene terephthalate) by Angelova, A. e.g., published in Khim Volokna 1978, (3), 19-20, the preparation of PETP-fibers with increased oxidative thermal stability are described by use of a mixture containing phenol- and phosphite-type compounds, such as Irganox 1222, abovementioned, and diglycol phosphate, triphenyl phosphite, or diphenyl isodecyl phosphite.
In the article `Oxidative thermal stabilization of PETP` by Angelova A, e.a., God. Vissh., Khim. Tekhnol Inst. Sofia 1983 (Publ 1984) 29(2), 123-6, the oxidative thermal stability of PETP is described when using a mixture of i.a. Irganox 1222, abovementioned, and triphenyl phosphite.
In the article `Effect of stabilisers on the preparation of PETP` by Chang, Shuya e.a., published in the J. Polym. Sci., Polym. Chem. Ed. 1982 20(8), 2053-61, the effect of stabilisation of a mixture of Irganox 1222, abovementioned, and triphenyl or trimethyl phosphates on the preparation of PETP is described.
In spite of the extensive prior art literature on the stabilisation of PETP-polymer, and the various solutions disclosed therein relating to the selection of specific stabilising additives, their time of addition to the reaction mixture. and various suggestions in respect of the molar ratio to be respected between the selected stabilizing compounds, the stabilization of the PETP-polymer, produced according to a continuous direct esterification and polycondensation process still is insufficient for the purposes of melt-extruding said PETP faultlessly to PETP-film suitable for being used as clear transparent support for photographic films.
The difficulty in producing PETP-polymer characterised by a high thermo-oxidative stability according to the continuous direct esterification and polycondensation process is particularly severe as the latter process becomes nowadays the state-of-the-art process for the production of PETP-polymer.
Indeed although various combinations of stabilising compounds have been described in the prior art literature, none of these combinations are suitable for producing PETP-polymer suitable for being used as support for photographic materials according to the continuous direct esterification and polycondensation process. On the one hand the high quality standards set forth for PETP-film sheets suitable for use as support for photographic materials, are different and/or more severe than the corresponding specifications for PETP-polymer intended for further processing to packaging materials, insulation materials injection molding materials or fibres. These specifications relate in particular to the color, the clarity and the transparency of PETP-polymer intended for further processing to sheets serving as support for photographic emulsions.
On the other hand the combination of stabilising compounds must conform to specifications set forth by the particulars of a continuous direct esterification and polycondensation process. Such specifications are e.g. the compatibility of the selected stabilising compounds with the polycondensation catalysts used and with the electroconductivity enhancing additives selected. The selected stabilising compounds must further not accumulate in any of the reactors or filters used in the continuous process, and must in general be compatible with the processing conditions in terms of temperature and pressure of such process. As the electroconductivity of the PETP-polymer produced is an important specification in view of the further processing through melt-extrusion of such PETP to film suitable as support for photographic materials, the selected stabilising compounds must not adversely affect the electroconductivity of the resulting PETP.
Finally the selected stabilising compounds must conform to environmental/ecological requirements in particular they must not cause any harmfull smell, and not be toxic. Last but not least, the selected stabilising compounds must be commercially available at reasonable conditions.
The specification in respect of clarity or color value of PETP-polymer, suitable for being melt-extruded to PETP-sheets for use as support for photographic materials, is as follows: the color value of PETP-polymer measured in accordance with the procedure described hereinafter, must be less or equal to 1.0.
The specification in respect of thermo-oxidative stability of PETP-polymer, suitable for being melt-extruded to PETP-sheets for use as support for photographic materials is as follows: the thermo-oxidative stability of PETP-polymer measured as a loss in weight in accordance with the procedure described hereinafter, must be less or equal to 0.35%.